Methods for forming three-dimensional ceramic and metallic articles having a predetermined size and shape are known. Most conventional of these methods are the various pressing and extrusion techniques and, for more complex shapes, injection molding. Because of the time needed to produce the tooling required for such processes and the high initial cost of that tooling, these production techniques typically require long lead times and result in high unit manufacturing costs, particularly for the manufacture of prototypes or for small quantity production runs.
Recently, various solid free form fabrication techniques have been developed for producing three-dimensional articles without the need for molds, dies or other tooling. One such technique, commercialized by Stratasys, Inc. of Eden Prairie, Minn., and referred to as Fused Deposition Modeling (FDM.TM.), builds solid objects layer by layer from polymer/wax compositions by following the signals generated by a computer-aided design (CAD) software program. According to this technique, described in U.S. Pat. No. 5,121,329, the disclosure of which is hereby incorporated by reference herein, a flexible filament of the polymer/wax composition is fed by a pair of counter-rotating rollers into a dispensing head including a liquifier and a nozzle outlet. Inside the liquifier, the filament is heating to a temperature just above its melting point where it softens and melts. As the counter-rotating rollers continue to advance the solid filament into the liquifier, the force of the incoming solid filament extrudes the molten material out from the nozzle where it is deposited on a build platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the vertical Z direction. By controlling the processing variables, the extruded bead, called a "road", can be deposited layer by layer in areas defined from the CAD model, leading to the creation of a three-dimensional object that resembles the CAD model.
As noted, the fused deposition modeling process described above has been used to fabricate three-dimensional objects from wax and polymer materials. The primary applications of this technology have been the manufacture of prototypes and the creation of positive forms to be used in investment casting processes. Accordingly, the users of the fused deposition modeling process have heretofore been concerned primarily with the external dimensional accuracy of the part formed and, to a lesser extent, with the exterior surface finish of the final part. As a result, little attention has been paid to the internal integrity of parts built by FDM techniques, and particularly to the elimination of internal defects which might limit the mechanical properties of solid parts built by these techniques. This is particularly the case in fabricating ceramic, metallic or ceramic/metallic parts, where the presence of internal defects such as voids, knit lines, laminations and the like will have a detrimental effect on the mechanical, electrical, optical, magnetic, magneto-optical and other properties of the final product.
The use of solid particle filled material in fused deposition modeling processes raises difficulties not addressed by the prior art. Among these concerns is the need to formulate solid particle filled feed materials having properties in both the solid and fluid forms which enable them to be fed into and deposited from the dispensing head, and the need to remove the binders from the formed article and sinter or otherwise densify the solid particles to from a final product having the desired dimensions and properties. The fabrication of solid functional components from these materials raises additional difficulties, including the need to avoid the occurrence of defects in the interior of the formed article.